[mojo-docs] Fix GN and mojom syntax errors

Fixes a few egregious errors in the doc examples.

Bug: None
Change-Id: If6d3ce3c9ff292c26558f5a5bf9a02d860b001b5
TBR: oksamyt@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539861Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#644364}

docs/mojo_and_services.md

@@ -64,7 +64,7 @@ module example.mojom;
 
 interface PingResponder {
   // Receives a "Ping" and responds with a random integer.
-  Ping() => (int random);
+  Ping() => (int32 random);
 };
 ```
 
@@ -73,7 +73,7 @@ definition here:
 
 ``` python
 # src/example/public/mojom/BUILD.gn
-import "mojo/public/tools/bindings/mojom.gni"
+import("//mojo/public/tools/bindings/mojom.gni")
 mojom("mojom") {
   sources = [ "ping_responder.mojom" ]
 }
@@ -327,7 +327,7 @@ interface Divider {
 
 ``` python
 # src/chrome/services/math/public/mojom/BUILD.gn
-import "mojo/public/tools/bindings/mojom.gni"
+import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojom") {
   sources = [