Fix outdated regexp of apache2 parser (#647)

The regexp shown in parser/apache2.md was the one used before v1.14.2.

Escape sequences were first allowed by df5ac352 "Allow escape sequences in
Apache access log" (v0.14.14), which used `(?:[^\"]|\\.)`, and that is the
form the document still shows. The source was later narrowed by db93f4b9
"parser_apache2: Fix too wide matching regexp" (v1.14.2):

    -      REGEXP = ... (?<path>(?:[^\"]|\\.)*?) ... (?<referer>(?:[^\"]|\\.)*) ... (?<agent>(?:[^\"]|\\.)*) ...
    +      REGEXP = ... (?<path>(?:[^\"]|\\")*?) ... (?<referer>(?:[^\"]|\\")*) ... (?<agent>(?:[^\"]|\\")*) ...

The document was not updated at that time. So it still says that any
backslash-escaped character is accepted in path, referer and agent, while
the current implementation accepts only an escaped double quote.

The new text is copied from Apache2Parser::REGEXP.source of the v1.19
branch and was compared with the document line to confirm they are
identical.

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

parser/apache2.md

@@ -11,7 +11,7 @@ See [Parse Section Configurations](../configuration/parse-section.md)
 Here is the regexp and time format patterns of this plugin:
 
 ```text
-expression /^(?<host>[^ ]*) [^ ]* (?<user>[^ ]*) \[(?<time>[^\]]*)\] "(?<method>\S+)(?: +(?<path>(?:[^\"]|\\.)*?)(?: +\S*)?)?" (?<code>[^ ]*) (?<size>[^ ]*)(?: "(?<referer>(?:[^\"]|\\.)*)" "(?<agent>(?:[^\"]|\\.)*)")?$/
+expression /^(?<host>[^ ]*) [^ ]* (?<user>[^ ]*) \[(?<time>[^\]]*)\] "(?<method>\S+)(?: +(?<path>(?:[^\"]|\\")*?)(?: +\S*)?)?" (?<code>[^ ]*) (?<size>[^ ]*)(?: "(?<referer>(?:[^\"]|\\")*)" "(?<agent>(?:[^\"]|\\")*)")?$/
 time_format %d/%b/%Y:%H:%M:%S %z
 ```