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.
diff --git a/README.md b/README.md
index 69bb3c5..c863b96 100644
--- a/README.md
+++ b/README.md
@@ -119,7 +119,7 @@
 If you’d like to build the documentation, it’s almost the same:
 
 ```sh
-./configure
+$ ./configure
 $ make docs
 ```