Idiomatic change to lowercase name in 'go get'

Author: JohnCGriffin

README.md

@@ -3,7 +3,7 @@
 Check for int/int8/int16/int64/int32 integer overflow in Golang arithmetic.
 ### Install
 ```
-go get github.com/JohnCGriffin/overflow
+go get github.com/johncgriffin/overflow
 ```
 Note that because Go has no template types, the majority of repetitive code is 
 generated by overflow_template.sh. If you have to change an