fetch-configlet: improve style slightly (#322)

This commit applies a preference for `==` over `=` when using `[[`.

From Google's Shell Style Guide [1]:

> For clarity, use `==` for equality rather than `=` even though both
> work. The former encourages the use of `[[` and the latter can be
> confused with an assignment.

We also remove the space before a function's parentheses to be more
consistent with the style of our other function, `get_download_url()`

[1] https://google.github.io/styleguide/shellguide.html#testing-strings

scripts/fetch-configlet

@@ -49,10 +49,10 @@ get_download_url() {
     cut -d'"' -f4
 }
 
-main () {
+main() {
   if [[ -d ./bin ]]; then
     output_dir="./bin"
-  elif [[ $PWD = */bin ]]; then
+  elif [[ $PWD == */bin ]]; then
     output_dir="$PWD"
   else
     echo "Error: no ./bin directory found. This script should be ran from a repo root." >&2