change the format of the linked issue number

Jorge Aparicio · Jorge Aparicio · commit 15fa301b7aa9 · 2017-04-07T09:46:55.000-05:00
diff --git a/src/doc/unstable-book/src/abi-ptx.md b/src/doc/unstable-book/src/abi-ptx.md
@@ -1,7 +1,8 @@
 # `abi_ptx`
 
-The tracking issue for this feature
-is: [38788](https://github.com/rust-lang/rust/issues/38788)
+The tracking issue for this feature is: [#38788]
+
+[#38788]: https://github.com/rust-lang/rust/issues/38788
 
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