Removed an invalid comment about I from README.md

`I` is a modifier, not a specifier; removed the invalid comment about it not having an "unsigned equivalent" - it can be used with unsigned specifiers.

Author: eyalroz

README.md

@@ -183,7 +183,6 @@ Notes:
 * The `%a` specifier for hexadecimal floating-point notation (introduced in C99 and C++11) is _not_ currently supported.
 * If you want to print the percent sign (`%`, US-ASCII character 37), use "%%" in your format string.
 * The C standard library's `printf()`-style functions don't accept `float` arguments, only `double`'s; that is true for this library as well. `float`'s get converted to `double`'s.
-* There is no unsigned equivalent of the `I` specifier at the moment.
 
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