Update char_tokenizer.py

[xiabingquan]

Fix this issue

In the method text2tokens of CharTokenizer. the space_symbol is set to "<space>" and can not be changed, but the right value should be self.space_symbol.

[kamo-naoyuki]

LGTM, thanks

[codecov]

Codecov Report

Merging #4499 (ccffc16) into master (2b00985) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4499   +/-   ##
=======================================
  Coverage   82.41%   82.41%           
=======================================
  Files         481      481           
  Lines       41237    41237           
=======================================
  Hits        33984    33984           
  Misses       7253     7253           

Flag	Coverage Δ
test_integration_espnet1	66.38% <ø> (ø)
test_integration_espnet2	49.17% <100.00%> (ø)
test_python	69.41% <100.00%> (ø)
test_utils	23.30% <ø> (ø)

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Impacted Files	Coverage Δ
espnet2/text/char_tokenizer.py	82.92% <100.00%> (ø)

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