Fix some code formatting in core_ext guide

[ci skip]

<plus> is nonstandard Textile; RailsGuides::TextileExtensions::plusplus
is the method that causes multiple code-formatted `+` symbols to
generate crap like

    <code>1 <notextile><tt>+</tt></notextile> 2 ...</code>

instead of the expected output:

    <code>1 + 2 ...</code>

guides/source/active_support_core_extensions.textile

@@ -1819,7 +1819,7 @@ NOTE: Defined in +active_support/core_ext/numeric/bytes.rb+.
 
 h4. Time
 
-Enables the use of time calculations and declarations, like 45.minutes + 2.hours + 4.years.
+Enables the use of time calculations and declarations, like @45.minutes <plus> 2.hours <plus> 4.years@.
 
 These methods use Time#advance for precise date calculations when using from_now, ago, etc.
 as well as adding or subtracting their results from a Time object. For example: