Add explanation for E0747

diff --git a/src/librustc_error_codes/error_codes/E0748.md b/src/librustc_error_codes/error_codes/E0748.md
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+A raw string isn't correctly terminated because the trailing `#` count doesn't
+match its leading `#` count.
+
+Erroneous code example:
+
+```compile_fail,E0748
+let dolphins = r##"Dolphins!"#; // error!
+```
+To terminate a raw string, you have to have the same number of `#` at the end
+than at the beginning. Example:
+```
+let dolphins = r#"Dolphins!"#; // one `#` at the beginning, one at the end so
+                               // all good!
+```