doctrenderer: make sure printf always uses a format string

[raboof]

Passing the string that is to be printed as the format string could lead to trouble when the string has sequences that would have meaning in a format string. Using the "%s" format string makes sure the string that is to be printed isn't interpreted as a format string.

This also makes compiling with -Werror=format-security happy.

[CLAassistant]

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