Based on conventions elsewhere, it is probably best to refer to it as the plus or + key.
The +/- dichotomy is used in other apps and more likely to be encountered by your users before they use yours even if it's in a different context. And your users are not trying to type the = character, they are trying to hit a specific key. This also allows users to use the + key on the num pad if their keyboard has one.
While it is true that some users could become confused and want to hold down shift, your software should be smart enough to take that into account and look at the key that is being pressed, rather than the character that is being typed.
For two quick examples, Zoom in Chrome and Font Size in Brackets (on a Mac) refer to using Cmd + and Cmd -. Granted, this is with a modifier key, but most apps that allow text input (in some capacity) use one so that you can do the function while in the middle of typing. This is something to consider in your app if a user might need to zoom or whatever while a text input has focus.
As always, testing with your actual users is the best way to know for sure. It could be that your audience skews heavily into the financial sector and they only press the buttons on the num pad (where = and + are separate keys) or maybe they are using Caps Lock and Shift for a lot of data entry or commonly typing = right before they have to change Zoom levels.
Worst case scenario: your users type on something that's not a standard QWERTY English keyboard and the + key can be in many different places. It may no longer make sense to +/- as a pair if they are nowhere near each other. Someone with more localization experience can hopefully weigh in on if keyboard shortcuts often get changed to support different language/keyboard conventions.