Change case of Explanation to match markdown (although it's functionally

equivalent)

common/lib/xmodule/xmodule/templates/problem/multiplechoice.yaml

@@ -29,10 +29,10 @@ metadata:
             ( ) The Beatles
             
             
-       [Explanation]
+       [explanation]
        The release of the iPod allowed consumers to carry their entire music library with them in a 
        format that did not rely on fragile and energy-intensive spinning disks.
-       [Explanation]
+       [explanation]
        "
 data: |
    <problem>

common/lib/xmodule/xmodule/templates/problem/numericalresponse.yaml

@@ -28,7 +28,7 @@ metadata:
        = 5
        
 
-       [Explanation]
+       [explanation]
        Pi, or the the ratio between a circle's circumference to its diameter, is an irrational number 
        known to extreme precision. It is value is approximately equal to 3.14.
                
@@ -37,7 +37,7 @@ metadata:
        can use any estimation technique that you like.
        
        If you look at your hand, you can count that you have five fingers.
-       [Explanation]
+       [explanation]
        "
 
 data: |

common/lib/xmodule/xmodule/templates/problem/optionresponse.yaml

@@ -17,12 +17,12 @@ metadata:
         [[(Multiple Choice), String Response, Numerical Response, External Response, Image Response]]
 
 
-        [Explanation]
+        [explanation]
         Multiple Choice also allows students to select from a variety of pre-written responses, although the 
         format makes it easier for students to read very long response options. Optionresponse also differs 
         slightly because students are more likely to think of an answer and then search for it rather than 
         relying purely on recognition to answer the question.
-        [Explanation]
+        [explanation]
         "
 data: |
   <problem>

common/lib/xmodule/xmodule/templates/problem/string_response.yaml

@@ -19,10 +19,10 @@ metadata:
         = Michigan
         
 
-        [Explanation]
+        [explanation]
         Lansing is the capital of Michigan, although it is not Michgan's largest city, 
         or even the seat of the county in which it resides.
-        [Explanation]
+        [explanation]
         "
 data: |
     <problem showanswer="always">