Some inappropriate uses of \text... to \math…

There are some inappropriate uses of `\text...` formatting commands – see e.g. Theorem 7.6.6 ("fact" is italicised) and Theorem 9.5.4 ("op" is italicised). The attached patch changes these to the corresponding `\math...` commands. (Another possibility is to insert an `\upshape` command somewhere to ensure that the text is always printed upright.)