Fix markup in RDoc [ci skip]

Author: kamipo

activesupport/lib/active_support/deprecation/behaviors.rb

@@ -54,7 +54,7 @@ class Deprecation
     # [+stderr+]  Log all deprecation warnings to <tt>$stderr</tt>.
     # [+log+]     Log all deprecation warnings to +Rails.logger+.
     # [+notify+]  Use +ActiveSupport::Notifications+ to notify +deprecation.rails+.
-    # [+silence+] Do nothing. On Rails, set `config.active_support.report_deprecations = false` to disable all behaviors.
+    # [+silence+] Do nothing. On Rails, set <tt>config.active_support.report_deprecations = false</tt> to disable all behaviors.
     #
     # Setting behaviors only affects deprecations that happen after boot time.
     # For more information you can read the documentation of the +behavior=+ method.
@@ -94,8 +94,8 @@ def disallowed_behavior
       #     # custom stuff
       #   }
       #
-      # If you are using Rails, you can set `config.active_support.report_deprecations = false` to disable
-      # all deprecation behaviors. This is similar to the `silence` option but more performant.
+      # If you are using Rails, you can set <tt>config.active_support.report_deprecations = false</tt> to disable
+      # all deprecation behaviors. This is similar to the +silence+ option but more performant.
       def behavior=(behavior)
         @behavior = Array(behavior).map { |b| DEFAULT_BEHAVIORS[b] || arity_coerce(b) }
       end