[rtext] Fix TextReplace() warning for const

[ghost]

Changes

1. Fixes the warning below (introduced after #3678):

rtext.c: In function ‘TextReplace’:
rtext.c:1537:17: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 1537 |     insertPoint = text;
      |                 ^

2. Does that by adding a cast to insertPoint = (char*)text; (R1537).

Edits:

• 1. added line marks.

[raysan5]

@ubkp thanks for the review!