Add license tags to documentation snippets.

docs/types/mapping-types.rst

@@ -41,6 +41,7 @@ contract that returns the value at the specified address.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.4.0 <0.7.0;
 
     contract MappingExample {
@@ -66,6 +67,7 @@ The example below uses ``_allowances`` to record the amount someone else is allo
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.4.22 <0.7.0;
 
     contract MappingExample {
@@ -120,6 +122,7 @@ the ``sum`` function iterates over to sum all the values.
 
 ::
 
+    // SPDX-License-Identifier: GPL-3.0
     pragma solidity >=0.6.0 <0.7.0;
 
     struct IndexValue { uint keyIndex; uint value; }