Bump MSRV for rustdoc:: lint prefix

Yes, this is stupid. But no, I don't want to implement a workaround.

See rust-lang/rust#86849

Author: jonhoo

.github/workflows/msrv.yml

@@ -10,10 +10,10 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: 1.49.0 # std::hint::spin_loop
+          toolchain: 1.52.0 # rustdoc:: lint prefix
           override: true
       - uses: actions/checkout@v2
-      - name: cargo +1.49.0 check
+      - name: cargo +1.52.0 check
         uses: actions-rs/cargo@v1
         with:
           command: check