mojo docs: Fix broken link in C++ bindings documentation

"Calling Mojo from Blink" was moved from a page in chromium.org to
docs/mojo_ipc_conversion.md in commit 4f3692b ("[mojo][doc] Add
blink-specific info"), adjust the link here accordingly.

TBR=oksamyt

Bug: 929385
Change-Id: Id00488a42c4bb3f514d3350c9d2badda084e271e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1591915
Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#655643}

mojo/public/cpp/bindings/README.md

@@ -1797,6 +1797,6 @@ See [Converting Legacy Chrome IPC To Mojo](/docs/mojo_ipc_conversion.md).
 
 ### Additional Documentation
 
-[Calling Mojo From Blink](https://www.chromium.org/developers/design-documents/mojo/calling-mojo-from-blink)
-:    A brief overview of what it looks like to use Mojom C++ bindings from
-     within Blink code.
+[Calling Mojo From Blink](/docs/mojo_ipc_conversion.md#Blink_Specific-Advice):
+A brief overview of what it looks like to use Mojom C++ bindings from
+within Blink code.