Update three links to point to main branch instead of master, removing the redirect (#2497)

On <https://wpt.fyi/results/> I've noticed when clicking on "[`view the search documentation`](https://github.com/web-platform-tests/wpt.fyi/blob/master/api/query/README.md)" you'll be taken to the documentation, but a banner from GitHub distracts you, telling you've been redirected to the default branch. This PR removes the redirect by directly linking to the main branch (which is the default). I've also changed two other occurrences. The redirect was acknowledged in #2280 (comment), but I thought it would be nicer without it.

Author: Schweinepriester

api/receiver/client/client.go

@@ -50,7 +50,7 @@ func (c client) CreateRun(
 	resultURLs []string,
 	screenshotURLs []string,
 	labels []string) error {
-	// https://github.com/web-platform-tests/wpt.fyi/blob/master/api/README.md#url-payload
+	// https://github.com/web-platform-tests/wpt.fyi/blob/main/api/README.md#url-payload
 	payload := make(url.Values)
 	// Not to be confused with `revision` in the wpt.fyi TestRun model, this
 	// parameter is the full revision hash.

webapp/components/test-search.js

@@ -360,7 +360,7 @@ class TestSearch extends WPTFlags(PolymerElement) {
              value="{{ queryInput::input }}" placeholder="[[placeholder]]"
              onchange="[[onChange]]" onkeyup="[[onKeyUp]]" onkeydown="[[onKeyDown]]" onfocus="[[onFocus]]" onblur="[[onBlur]]">
       <span class="help">
-        For information on the search syntax, <a href="https://github.com/web-platform-tests/wpt.fyi/blob/master/api/query/README.md">view the search documentation</a>
+        For information on the search syntax, <a href="https://github.com/web-platform-tests/wpt.fyi/blob/main/api/query/README.md">view the search documentation</a>
       </span>
 
       <!-- TODO(markdittmer): Static id will break multiple search components. -->

webapp/components/wpt-flags.js

@@ -312,7 +312,7 @@ class WPTEnvironmentFlagsEditor extends FlagsEditorClass(/*environmentFlags*/ tr
     <paper-item sub-item>
       <paper-checkbox checked="{{onlyChangesAsRegressions}}">
         Only treat C (changed) differences as possible regressions.
-        (<a href="https://github.com/web-platform-tests/wpt.fyi/blob/master/api/README.md#apidiff">See docs for definition</a>)
+        (<a href="https://github.com/web-platform-tests/wpt.fyi/blob/main/api/README.md#apidiff">See docs for definition</a>)
       </paper-checkbox>
     </paper-item>
     <paper-item>