[tex-k] typos in errata.tex

[胡亚捷 (Hu Yajie)]

Knuth did the Pascal-to-TeX translation in errata.tex by hand and introduced
some errors. Here are two obvious ones:

errata.tex, lines 1017--8:
@x
\quad{\bf define} $\\{fi\_or\_else}=2$\quad$\{\,$delimiters for conditionals
  {\bf elseif\/}, {\bf else}, {\bf fi})$\,\}$
@y
\quad{\bf define} $\\{fi\_or\_else}=2$\quad$\{\,$delimiters for conditionals
  ({\bf elseif\/}, {\bf else}, {\bf fi})$\,\}$
@z

errata.tex, line 1575:
@x
 \.{\char`"charcode:=charcode+code\_offset;"}$;
@y
 \.{\char`"charcode:=charcode+code\char`\_offset;"}$;
@z

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Previous reports:
https://github.com/dine2014/texbook-mfbook-bugs/blob/master/bugs.md