docs($http): fix typo in `headersGetter`

Closes #15198

src/ng/http.js

@@ -195,7 +195,7 @@ function parseHeaders(headers) {
  * @param {(string|Object)} headers Headers to provide access to.
  * @returns {function(string=)} Returns a getter function which if called with:
  *
- *   - if called with single an argument returns a single header value or null
+ *   - if called with an argument returns a single header value or null
  *   - if called with no arguments returns an object containing all headers.
  */
 function headersGetter(headers) {