Fix README.md command consistency

The ./configure command in README.md's Building Documentation section was
missing the $ prefix. Add the prefix to be consistent with other commands in the
document.

Author: jviide

README.md

@@ -119,7 +119,7 @@ $ make && make install
 If you’d like to build the documentation, it’s almost the same:
 
 ```sh
-./configure
+$ ./configure
 $ make docs
 ```