The technical term is indeed a tear notch as mentioned by Lambie
Greener Corporation in Bayville, New Jersey, USA say
A tear notch makes packages produced on vertical form/fill/seal baggers easier to open. This small slit in the end seal allows consumers to easily tear the film in order to gain access to the product.
The slit is produced by a carbide notch punch that is affixed in the sealing face of the jaw. The carbide material provides exceptional durability, and the jaw mounted design provides significant cost benefits when compared to notch punches that are attached to the machine knife.
The tear notch can be located wherever desired on the end seal of the bag and can be integrated with a tear strip feature or “Open Here” graphics.
Greener corporation mentions a slit
slit: a straight, narrow cut or opening in something
however, the Merriam-Webster dictionary describes slit as
a long narrow cut or opening
Notches on packaging are generally small
You could possibly call it a nick
nick: a small notch, groove, or chip
Turpack, a company which manufactures packaging machines advertises nick knives as a feature on their machines. Referring to tear notches, they say
It is a small cut that extends to the adhesive bead in the edge of the disposable packaging. The package is torn, starting from the notch in order to open it. The product is easily emptied from the formed cavity. Foods, medical products and beverages reach the customer extremely simply and quickly with easy open tear notch. Apart from this, we can produce serrated notch and laser notch.
The type of packet shown in the question is known as a sachet.
Alibaba.com advertises a number of them. Larger sachets are called pouches.